The present invention generally relates to storage units, optical recording mediums and information recording methods, and more particularly to a storage unit for reproducing information which is recorded with a high density from a recording medium, an optical recording medium recorded with information with a high density, and an information recording method for recording information on a recording medium at with a high density.
As one type of optical recording medium, there is the magneto-optical recording medium typified by a magneto-optical disk. The magneto-optical disk has a substrate, and a recording layer made of a magnetic material and formed on the substrate, and records information using changes in magnetic field and heat caused by light. In addition, a magneto-optical effect is used when reproducing information from the magneto-optical disk. A data track for recording data, and a control track for recording medium information peculiar to the magneto-optical disk are provided on such a magneto-optical disk, where each track includes an identification (ID) part for identifying a sector which is a recording region and a data part for recording the data. In order to prevent rewriting of the information, the manufacturer of the magneto-optical disk records the control track by forming concavo-convex parts (embossed pits) on the substrate by use of a stamper or, records the control track simultaneously as the formation of guide grooves (lands/grooves) on the substrate by injection molding. For similar reasons, the ID part is recorded by forming the concavo-convex parts on the substrate by the same production process.
Various methods have been proposed conventionally to improve the recording density of the magneto-optical disk described above. According to one method which uses the magnetic super resolution (MSR), although the minimum recorded information which can be reproduced is generally determined by the wavelength, it is possible to reproduce information smaller than such a limit. In other words, by forming a magnetic mask by utilizing a temperature distribution of a laser power at the time of the reproduction, it is possible to reproduce only the necessary information from the magneto-optical disk.
FIG. 1 is a diagram for explaining the operating principle of the method which uses the MSR. In FIG. 1, the upper part shows a plan view of a part of 1 track on the magneto-optical disk, and the lower part shows a cross sectional view of the magneto-optical disk. A recording layer 101, an intermediate layer 102 and a reproducing layer 103 are provided on a substrate (not shown) of the magneto-optical disk. Arrows within these layers 101 through 103 indicate the magnetization direction. In FIG. 1, BM denotes a moving direction of the laser beam, DM denotes a moving direction (rotating direction) of the magneto-optical disk, RM denotes a reproducing magnetic field, and the hatching indicates an interface magnetic domain wall 104.
The intermediate layer 102 transfers or blacks the information recorded in the recording layer 101 to the reproducing layer 103 depending on the temperature. When reproducing the information recorded in the data part of the track in this manner, the temperature distribution of the laser power at the time of the reproduction is utilized to form a magnetic front mask 105 and a rear mask 106 at parts other than the reproducing position, so that it is possible to reproduce only the necessary information from the magneto-optical disk. In other words, in a case where the information recorded in the data part has a minimum mark length of 0.38 xcexcm and this information is reproduced using a laser beam having a wavelength of 680 nm, for example, it is possible to reproduce only the necessary information from the magneto-optical disk by forming the masks 105 and 106, even if the spot diameter of the laser beam is approximately 1 xcexcm and is approximately 3 times the minimum mark length.
However, the ID part of the control track is recorded by forming the concavo-convex parts (embossed pits) on the substrate of the magneto-optical disk. For this reason, even if an attempt is made to record the information in the ID part of the control track with the same density as the data part of the control track, the MSR cannot be used, and there was a problem in that the information recorded in the ID part cannot be reproduced accurately. In other words, in the case where the spot diameter of the laser beam is approximately 1 xcexcm as described above, for example, approximately 3 pits fall within the beam spot even if an attempt is made to reproduce the pit having the minimum mark length of 0.38 xcexcm, and it is possible to reproduce the information from only the necessary one of the 3 pits. This is because there is no known means for masking the information from the pits other than the necessary pit from among the 3 pits which fall within the beam spot.
Accordingly, it is a general object of the present invention to provide a novel and useful storage unit, optical recording medium and information recording method in which the problems described above are eliminated.
Another and more specific object of the present invention to provide a storage unit, an optical recording medium and an information recording method which can accurately reproduce information recorded in the form of concavo-convex parts, even when reproducing the information from the recording medium by the magneto-optical effect utilizing the MSR.
Still another object of the present invention is to provide a storage unit for optically reproducing information from an optical recording medium of a type having a control region which is prerecorded with control information with an embossed shape and a data region in which data is recorded by an optical means,.where the control information includes medium information peculiar to the recording medium, the control region and the data region have mutually different recording densities, and the storage unit comprises control means for switching a frequency of a read clock which is used when reproducing the control information and the data from the optical recording medium between a case where the control information is reproduced and a case where the data is reproduced. According to the storage unit of the present invention, because the data part and the identification part of the control region are both prerecorded with the embossed shape, it is possible to simplify the control of the read clock frequency, by making the read clock frequency the same for the data part and the identification part of the control region. For this reason, even if the recording density of the data region increases, it is possible to read the control information from the control region.
In the storage unit, the recording density of the control region may be 1/N times the recording density of the data region, where N greater than 1.
In addition, in the. storage unit, the control means may switch the frequency of the read clock when reproducing the control information to 1/N times the frequency of the read clock at a time of reproducing the data, where N greater than 1.
In these cases, when the read clock frequency for reading the data part of the data region is set to 1/N times the read clock frequency for reading the identification part of the data region and the control region, it becomes possible to obtain the required read clock frequencies with a high accuracy.
On the other hand, the storage unit may further comprise a first generator which generates a reference clock, and a second generator which generates first and second clocks based on the reference clock, where the control means switches the frequency of the read clock to a frequency of the first clock when reproducing the control information from the control region, and to a frequency of the second clock when reproducing the data from the data region. In this case, by generating the first and second clocks based on the reference clock, it is possible to possible easily synchronize the first and second clocks. In addition, the first and second clocks can easily be generated by dividing the reference clock by different frequency dividing ratios, thereby making it unnecessary to provide a plurality of clock generating circuits, and the circuit scale is simplified.
In this case also, the frequency of the first clock may be 1/N times the frequency of the second clock, where N greater than 1. In this case, when the read clock frequency for reading the data part of the data region is set to 1/N times the read clock frequency for reading the identification part of the data region and the control region, it becomes possible to obtain the required read clock frequencies with a high accuracy.
A further object of the present invention is to provide an optical recording medium comprising a control region prerecorded with control information with an embossed shape, and a data region recorded with data by an optical means, where the control region has a recording density which is 1/N times a recording density of the data region, where N greater than 1. According to the optical recording medium of the present invention, because the data part and the identification part of the control region are both prerecorded with the embossed shape, it is possible to simplify the control of the read clock frequency, by making the read clock frequency the same for the data part and the identification part of the control region. For this reason, even if the recording density of the data region increases, it is possible to read the control information from the control region.